A sticky substance is by-produced in a reaction process of petrochemical process, and waste liquid containing the sticky substance is extremely difficult to be removed due to its stickiness. Usually, the sticky substance is removed by methods such as dissolving by a solvent. In a process of producing and storing an easily-polymerizable substance, a partially polymerized substance is generated. The process liquid containing the partially polymerized substance (sticky substance) is also removed as waste liquid, usually, by methods such as dissolving by a solvent (see, Japanese Patent Application Laid-Open (JP-A) No. 2000-290225). If the stickiness is lower to a certain extent, the waste liquid is fed as it is to an incinerator by a pump without using a solvent, and then it is burned. The steam is recovered from waste liquid, thus, waste liquid can be treated economically and efficiently in some cases.
However, if the stickiness is higher to a certain extent, a larger amount of solvent is generally necessary as compared with the amount of waste liquid. Therefore, cost required for a solvent itself, and deterioration in working environments because of harmfulness and volatility of the solvent, are often problematical. It is apparent that recovering a large amount of solvent after dissolution and burning this gives energy waste. Particularly, process liquid containing a polymerized substance generated in an equipment for producing methyl (meth)acrylate and (meth)acrylic acid contains by-products having high stickiness and showing solid-like nature in some cases. Among them, there are not a few substances poorly-soluble in a solvent. Such process liquid is impossible to be lifted by a pump in some cases.
In the specification of the instant application, (meth)acrylic acid is generic name for acrylic acid and methacrylic acid. Methyl (meth)acrylate is generic name for methyl acrylate and methyl methacrylate.
Removal of liquid containing such a sticky substance from a vessel is disadvantage from the standpoints of cost, working environment or energy saving in conventional methods using a solvent, and complete removal thereof is difficult.